Damascus, January 13 (RHC)-- Witnesses in Syria say a recent U.S.-led coalition airstrike may have killed dozens of civilians. According to McClatchy News, the attack hit a makeshift Islamic State jail in the town of Al-Bab close to the Turkish border on December 28th. Most of the reported dead were civilian prisoners held by the Islamic State for violations of Sharia law or opposition fighters from rival groups.

The Syrian Network for Human Rights says it believes around 50 civilian prisoners died, which would make it the worst attack on civilians in Syria since the U.S.-led campaign began. The Pentagon says it has no evidence of civilian casualties so far. But it also acknowledged it failed to disclose the Al-Bab attack in its weekly summaries, and only confirmed it following a request from McClatchy reporters.

Military officials have recently announced a review of civilian deaths in the U.S.-led bombing campaign, but have only acknowledged a small number of killings to date. According to Pentagon figures, the U.S.-led coalition carried out 34 strikes in Iraq and Syria in recent days, 25 in Syria and nine in Iraq.